Unnumbered (yellow) bookmarks lost

I just had again the problem that all my unnumbered (yellow) bookmarks are lost. I had that problem already many times in the past. I know that these bookmarks are saved in the appropriate *.ppr-file (Mark#=Col,Line). Is there anything I can do to avoid that? At the moment, I'm using PSPad versionb 4.5.7 (2450).

Many thanks and greetings
padFox